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Characterization of surface change properties of synthetic microfiltration membranes by means of a streaming current method
Desalination, 79 (1990) 271481 Elsevier Science PublishersB.V.,
271
Amsterdam
Characterization of Surface Change Properties of Synthetic Microfiltration Membranes by Means of a Streaming Current Method H. ZHANG and U. WBRNER LAtstuhl fiir Mechanische Verfahmstechnik, UniversitiirDorbnund, D-4600 Dortmund SO, (Germany) (Received October 29,1989; in revisedfom Febrwuy $1991)
SUMMARY
By use of an improved streaming current method we characterized the pH dependent surface charge properties of six synthetic microfiltration membranes at the constant electrolyte concentration of 10” mol/l. The observed pH dependence of the surface charge properties of the cellulose acetate and the nylon 66 membranes could be explained by the pH influence on the dissociation and protonation of the surface end groups of membrane materials, while the reaction of the surface modification agents, i.e., the wetting agents, with the electrolytes and the ion adsorption on the membrane surfaces, were the’ main causes for the surface charge of nonionic polymer membranes (polyvinylidenfluoride and polysulfone membranes) in the solutions which had different pH values.
INTRODUCTION
Membrane separation processes, such as microfiltration and ultrafiltration processes, are often used in the food industry, biotechnology, the pharmaceutical industry and waste water treatment. Hereto the electric potential of membrane surfaces plays a very important role for the improvement of membrane filtration efficiency. Knight et al. [l] demonstrated photographically, compared with unmodified nylon membranes, that an enhanced removal of negatively charged latex particles with a particle size below the ooll-9164/90/$03.50
0 1990 Elsevier Science PublishersB.V.
272
pore size of membranes is observed in the filtration processes with modified cationic charged nylon membranes. The same effect could also be utilized for pyrogen control in a pharmaceutical process [2]. Even the membrane fouling, which is caused mainly by hydrodynamic forces under filtration, concentration polarization and interactions between membrane surface and particles could be reduced by an electrostatic repulsive force between membrane surface and particles [3,4]. The streaming current, streaming potential and electro-osmosis measurements are the electrokinetic measurements used for the determination of the surface electric potential of porous plugs placed in electrolytes [5]. They were also applied for the investigation of the surface charge properties of synthetic microfiltration and ultrafiltration membranes [6-lo]. The purpose of this paper is to demonstrate the successful application of an improved streaming current method [ 11,121to the investigation of surface charge properties of synthetic microfiltration membranes. This method was introduced by van der Put et al. [ 11,121 for the electrokinetic measurements of polystyrene plugs. Compared with the results of the normal streaming current measurements, the results of the improved method are not falsified by electrode polarization usually observed in the normal streaming current measurements. Furthermore, the results of such measurements on six synthetic microfiltration membranes at various pH values are discussed.
THEORETICAL BACKGROUND The surface of substances becomes charged if the substances are brought into contact with a polar (i.e., aqueous) medium. Two of the possible charging mechanisms are the adsorption of ions or polyelectrolytes of charged macromolecular species and the dissociation of surface groups [6]. In addition to the mixing tendency of thermal motion, these surface electric charges influence ions in the polar medium nearby in such a way that an electric double layer is formed [13]. This electric double layer consists of the charged surface and a diffuse region with an excess of counter-ions over coions (processing unlike and like charges to the surface charges). Due to the electroneutrality of the system, these surface charges are compensated by the charges of ions (counter-ions) in the diffuse part [13]. During a streaming current measurement on a microfiltration membrane, an electrolyte is forced by a pressure difference to flow through the pores of the membrane. In this case the electric double layer in the boundary phase of the surface of the membrane pores and the solution will be sheared. The net charge flow (streaming current I,) of ions contained in the electric
273
double layer can be recorded by two electrodes connected to an amperemeter and can be converted into the electric potential (zeta-potential C) at the surface of shear which has a distinct distance to the charged membrane surface. For the case of a laminar flow through a single circular capillary with diameter d and length 1, the equation which describes the relationship between the streaming current I, and the zeta-potential C can be written in the form [13]: (1) where E,, is the permitivity of the vacuum, e, the relative dielectric constant of the solution, Ap the applied pressure difference between the ends of the circular capillary and 11the viscosity of medium. As the volumetric flow rate 3 with the average velocity urn through the capillary obeys the Poiseuille’s equation [ 131, Eqn. (1) can be rewritten as follows [ 141: I, = -8xe,e,Cu,
32e,e,C
.
=-TV
(2)
Eqn. (2) can also be applied to a microfiltration membrane if the membrane is regarded as a bundle of circular capillaries with an average pore size that is larger than the thickness of the electric double layer appearing in the membrane pores.
EXPERIMENTAL
Principle of the measurement Since no electrode is completely reversible to any electrolyte, the charge flow through the microfiltration membrane, compared to the charge flow purely generated by the pressure difference, is more or less reduced by an electrode polarization occurring at the two electrodes employed in a normal streaming current measurement. For this reason the results of the normal streaming current measurements are in most cases falsified. However, utilizing the electrode polarization, we can obtain the exact streaming current by means of a four-electrode technique [ 111.The arrangement of the four electrodes employed in the improved streaming current measurement is shown schematically in Fig. 1.
274
El
Fig. 1. The electrode arrangement in the improved streaming current measurement: El, El’, the electrodes for potential measurement; E2, E2’, the electrodes for current measurement; M, the microfiltration membrane; C, the measuring cell; 0, O-ring seal.
Just like in the normal streaming current measurements, the two inner electrodes (E2, E2’) neighboring the membrane are used for the current measurement. The electrode polarization occurring at the two inner electrodes is observed with the two external electrodes (El, El’) by means of potential measurement. The charges of the ion current generated by the pressure difference are either digested by the electrodes (E2, E2’) or conductively transported backwards by the electric potential at the electrodes (E2,E2’) through the membrane. In the case of an application of an abrupt and constant pressure difference Apl to the systems, the temporal development of the measured quantities I and U is demonstrated in Fig. 2. The electric current recorded by the inner electrodes (E2, E2’) is reduced gradually from the streaming current while the electric potential measured by the external electrodes is increased continuously. Concerning the measured quantities I and U, the differences between the ideal case (streaming current measurement without electrode polarization) and the real case (streaming current measurement with electrode polarization) will meet finally to the constant values 1,-I.. and U&J_. The relationship between the time-dependent values I and U can be expressed by the equation of thermodynamics of irreversible processes [ 111:
275 I = LIAp-LzU
(3)
where L, and L, are the phenomenological
0 tl
coefficients.
t
Fig. 2. The temporal development of the measured quantities in an improved streaming current measurement: IO, I and I, - electric current flow through the electrodes (E2, E2’) at time 0, t and a~; U,, U and U, - electric potential at the electrodes (E2, E2’) at time 0, t and -; Zs - streaming current, if Z,-,is defined as the “zero” current. I ls2\
uo
U
Fig. 3. Linear relationship between timedependent Z and U under two pressure differences Apl and Apz. I0 and U, - electric current and potential at the time of 0; Is1 and Zs2 - streaming currents under pressure Ap1 and Ap2.
276
If the measured values of the current I are plotted down continuously vs. the measured values of the potential Uduring a measurement, a straight line will be obtained (Fig. 3). The extrapolation of one of the straight lines to the “zero” potential (U,) yields the streaming current I, with the classical definition [ 111.A change of the values of the applied pressure difference from Apl to Ap2 causes only a parallel line against the straight line obtained under the pressure difference APl* Materialsand chemicals AU microfiltration membranes used in this study had a nominal pore size of 0.2 pm. They were Millipore GVWP (polyvinylidenfluoride), Brunswick BTSD (polysulfone), Sartorius SMllO (cellulose acetate), Sartorius SM200 (nylon 66), Pall Ultipor NR (nylon 66) and Pall Posidyne NAZ (modified nylon 66). In order to obtain the same thickness of the electric double layer in the boundary phase of a membrane surface and the electrolytes, we kept the cation and anion concentration of the electrolytes used at lOA mol/l in the scope of pH 4 to pH 10. We prepared the electrolytes by use of distilled water adjusted with HCl, NaOH and HCl. Apparatus The apparatus for the improved streaming current measurement is shown in Fig. 4. A microfiltration membrane (M) was placed between two platinized platinum mesh electrodes in a plexiglass measuring cell (C) which was connected to two electrolyte reservoirs (R). In the electrolyte reservoirs there were two platinized plate electrodes. The surface of the mesh electrodes should be treated in such a way that in the electrolytes used an electrode polarizations at these electrodes occurred at a certain time during streaming current measurement. If a pressure difference was applied to the electrolyte reservoirs, the electrolyte flowed through the membrane. The values of the current and the potential measured by the four electrodes were amplified and recorded by an X-Y plotter (PL). The electrolyte flux through the membrane due to the pressure difference is determined by a balance (W) after each streaming current measurement. Before a streaming current measurement was started, the membrane was first degassed and then swelled in the electrolyte outside the cell for 24 h.
277
After that the membrane was washed for 20 min under a pressure difference of 0.1 bar in the cell by filtering the electrolyte through it.
CA
Fig. 4. Streaming current apparatus: A, amplifiers; PL, X-Y plotter; V, voltmeter; M, membrane; CA, compressed air; P, pressure sensor; E, electrodes; B, buffer; C, measuring cell; W, balance; RC, recorder.
All streaming current measurements were accomplished by pressure differences under 0.12 bar. During a measurement, the linear proportionality of the streaming current I, and the volumetric flow rate V of the electrolyte to the applied pressure difference Ap was checked for the membrane used.
RESULTS AND DISCUSSION
As the microfiltration membranes used had different structures with regard to the membrane porosities, the shapes of the membrane pores and the structures across a membrane thickness, it is very difficult to find the correct pore sizes of the membranes to evaluate the membrane surface potential (zeta-potential C) according to Eqn. (2). The surface charge properties of the microfiltration membranes were, therefore, characterized simply by the quotient of the measured streaming current I, and the volumetric flow rate 3 of the electrolytes.
278
Fig. 5 shows the determined quotients I,lvplotted vs. the pH values of the electrolytes for the cellulose acetate membrane (Sartorius SMllO) and the two native nylon 66 membranes (Pall Ultipor NR and Sartorius SM200).
4
5
6
7
8
9
10
PH
Fig. 5. pH influences on the surface charge of three membranes. a - Sartorius SMllO; b - Pall Ultipor NR; c - Sartorius SM200; Electrolyte concentration ccatioda~on = 10s4 mol/l.
The pH impact on the surface charge of these three membranes can be explained by the reactions of the membrane materials with the electrolytes if the ion adsorption on the membrane surfaces is negligible. On the surface of the cellulose acetate membrane (Sartorius SMllO), there were probably some groups which were able to dissociate as a weak acidic group. The number of these groups might be influenced by the hydrolysis of cellulose acetate. Thus the surface charge of the cellulose acetate membrane increased with the rise of the pH value of the electrolytes. At the high pH all weak acidic groups of the cellulose acetate membrane were dissociated, and, therefore, its surface potential remained constant. This effect has also been discussed theoretically by Demisch and Pusch [ 151.Their calculations of the surface charge of cellulose acetate membranes in the range of pH 4-10 delivered results that showed the same development as our experimentally determined results. The surface charge property of two native nylon 66 membranes (Pall Ultipor NR and Sartorius SM200) was much more influenced by the pH value of the electrolytes because many amino groups and carboxyl groups exist on the membrane surfaces. At a high pH value the surface charge of the membranes was mainly caused by the dissociation of the carboxyl groups which acted in the solutions as a weak acidic group. If the pH value of the electrolytes were decreased, the dissociation of the carboxyl groups dimin-
279
ished and the protonation of the amino groups increased. Therefore, with the decrease of the pH value the surface potential of the two native nylon 66 membranes became more positively charged that the enhanced negative values of the quotients IJvwere recorded. The similar pH influence on the surface potential of protein was reported by Shaw [13]. The difference of the surface charge properties between the Sartorius SM200 and Pall Ultipor might be dependent on the different ratios of the ammo groups to carboxyl groups existing on the surfaces of the two microfiltration membranes. Fig. 6 shows the results obtained by the streaming current measurements on the membranes which were modified. The modifications were made either on the membrane material (Pall Posidyne NAZ) or on the membrane surface (Millipore GV,WP and Brunswick BTSD). 750 600 GO
-600
I I L
5
6
7
6
9
l0
PH
Fig. 6. pH influences on the surface charge of three membranes: d - Millipore GVWP; e Brunswick BTSD; f - Pall Posidyne NAZ; Electrolyte concentration Ccation/anion= lob4 mol/l.
The typical amphoteric property of nylon 66 membrane could be quite well observed at the Pall Posidyne NAZ membranes whose surface potential kept positively charged in the whole investigated pH range by building of some chemical end groups in this membrane. These chemical end groups could be protonated to the quaternary ammonium end groups and enhanced the number of positively charged end groups on the nylon 66 membrane surface. The surfaces of the membranes made of hydrophobic materials, such as the Millipore GVWP and Brunswick BTSD membrane were always modified to improve the wettability of the membranes. As the hydrophobic materials do not have any ionic end groups which may cause surface charge of these membranes, the reason for the measured surface charge properties can only
be explained as a result of the ion adsorption on the membrane surface [16] and/or the reaction of the wetting agents with the electrolytes. The latter case is more convincing since positively and negatively charged polyvinylidenfluoride membranes have been offered by a membrane manufacturer at the same time. The pH dependence of the surface charge of the polysulfone membranes (Brunswick BTSD) is similar to the results reported for other polysulfone membranes [6,8]. Unfortunately, in one case [6] different chemicals besides HCl and NaOH were used to adjust different pH values, whereas in another case [B] the ion concentration of the electrolytes used was changed in several orders. For this reason a comparison of our results with those measured values is impossible. Although it was assumed by both authors that the ion adsorption, for example the adsorption of hydroxyl ions, is the main cause for the surface potential of polysulfone, we do not exclude other possibilities to impact the surface potential of the polysulfone membranes. One such possibility is the reaction of the surface modification agents (the wetting agents) with the electrolytes used. This might be the main reason for the negative surface charge of the polyvinylidenfluoride membrane (!&llipore GVWP). The existence of such reactions on the surface of this membrane in the electrolytes with a certain pH value was not denied by the membrane manufacturer. Nevertheless, we believe that the behavior of the surface charge of these two membranes in the electrolytes with various pH values should be an outcome of the combination of the ion adsorption and the reaction of the wetting agents on the membrane surface with the electrolytes. A general description of the surface charge properties of membranes made of such materials could hardly be made; because of the use of different wetting agents, their surface charges can be changed in different ways.
CONCLUSIONS
As we used the improved streaming current method, the surface charge properties of the synthetic microfiltration membranes were described by the measured quantities. Compared to the results of the normal streaming current measurements, the results of these improved streaming current measurements were not falsified by the electrode polarization. The shown pH dependency of the surface charge properties of the cellulose acetate membrane and the nylon 66 membranes could be explained by the pH influence on the dissociation and protonation of the surface end groups of the membrane materials. On the contrary, the surface reaction of the surface
281
modification agents,i.e., the wetting agents, with the electrolytes and the ion adsorption on the membrane surface were probably the main causes of the surface charge of the nonionic polymer membranes in the electrolytes with various pH values.
ACKNOWLEDGEMENT
The authors wish to record their gratitude to the BMWi of the Federal Republic of Germany for financial support. This project is a part of the industrial joint research work of BMWi via DECHEMA.
REFERENCES 1 RA. Knight, J.V. Fiore and J.V. Rositto, World Filtration Congress III, Downington, Pa., 1982, p. 427. 2 T.H. Meltzer, Filtration in the Pharmaceutical Industry, Marcel Dekker, New York, 1987, p. 204. 3 H.P. Gregor, Fixed-charge ultrafiltration membranes, in: E. S616gny (Ed.), Charged Gels and Membranes I, D. Reichel, Dordrecht-Holland, 1976. 4 J. Nassauer and H.C. Kessler, Chem. Ing. Tech., 58 (1986) 244. 5 M.V. Smoluchlowski, Elektrische Endosmose und Stromungstrdme, in: Handbuch der Elektrizitat und das Magnetismus, Bd. II, Graetz, Leipzig, 1921. 6 C.K. Lee and J. Hong, J. of Membr. Sci., 39 (1988) 79. 7 M. Nystrom, M. Lindstrom and E. Matthiasson, Colloids and Surfaces, 36 (1989) 297. 8 J. Nassauer and H.G. Kessler, ZFL (June 1984) 484. 9 J. Benavente and C. Femandez-Pineda, J. Membr. Sci., 23 (1985) 121. 10 J.A. IbaAez, J. Forte, A. Hemandez and F. Tejerina, J. Membr. Sci., 36 (1988) 45. 11 A.G. van der Put, Electrokinetic investigation on the system polystyrene/aqueous electrolyte solution. Ph.D. Thesis, Agricultural University, Wageningen, 1980. 12 J.J. van den Hoven, Electrokinetic properties and conductance relaxation of polystyrene and silver iodide plugs. Ph.D. Thesis, Agricultural University, Wageningen, 1984. 13 D.J. Shaw, Introduction to Colloid and Surface Chemistry, Butterworth, Boston, 1980. 14 W. Matzmorr, Elektrokinetische Untersuchung an ladungsmodifiiierten Filterschichten. Dissertation, Universitat Dortmund, 1988. 15 H.U. Demisch and W. Pusch, J. Electrochem. Sot., Electrochemical Sci. Tech. (March 1976) 370. 16 C.J.O.R. Morris and P. Morris, Separation Methods in Biochemistry, 2nd Ed., Halsted Press, New York, 1976.
271
Amsterdam
Characterization of Surface Change Properties of Synthetic Microfiltration Membranes by Means of a Streaming Current Method H. ZHANG and U. WBRNER LAtstuhl fiir Mechanische Verfahmstechnik, UniversitiirDorbnund, D-4600 Dortmund SO, (Germany) (Received October 29,1989; in revisedfom Febrwuy $1991)
SUMMARY
By use of an improved streaming current method we characterized the pH dependent surface charge properties of six synthetic microfiltration membranes at the constant electrolyte concentration of 10” mol/l. The observed pH dependence of the surface charge properties of the cellulose acetate and the nylon 66 membranes could be explained by the pH influence on the dissociation and protonation of the surface end groups of membrane materials, while the reaction of the surface modification agents, i.e., the wetting agents, with the electrolytes and the ion adsorption on the membrane surfaces, were the’ main causes for the surface charge of nonionic polymer membranes (polyvinylidenfluoride and polysulfone membranes) in the solutions which had different pH values.
INTRODUCTION
Membrane separation processes, such as microfiltration and ultrafiltration processes, are often used in the food industry, biotechnology, the pharmaceutical industry and waste water treatment. Hereto the electric potential of membrane surfaces plays a very important role for the improvement of membrane filtration efficiency. Knight et al. [l] demonstrated photographically, compared with unmodified nylon membranes, that an enhanced removal of negatively charged latex particles with a particle size below the ooll-9164/90/$03.50
0 1990 Elsevier Science PublishersB.V.
272
pore size of membranes is observed in the filtration processes with modified cationic charged nylon membranes. The same effect could also be utilized for pyrogen control in a pharmaceutical process [2]. Even the membrane fouling, which is caused mainly by hydrodynamic forces under filtration, concentration polarization and interactions between membrane surface and particles could be reduced by an electrostatic repulsive force between membrane surface and particles [3,4]. The streaming current, streaming potential and electro-osmosis measurements are the electrokinetic measurements used for the determination of the surface electric potential of porous plugs placed in electrolytes [5]. They were also applied for the investigation of the surface charge properties of synthetic microfiltration and ultrafiltration membranes [6-lo]. The purpose of this paper is to demonstrate the successful application of an improved streaming current method [ 11,121to the investigation of surface charge properties of synthetic microfiltration membranes. This method was introduced by van der Put et al. [ 11,121 for the electrokinetic measurements of polystyrene plugs. Compared with the results of the normal streaming current measurements, the results of the improved method are not falsified by electrode polarization usually observed in the normal streaming current measurements. Furthermore, the results of such measurements on six synthetic microfiltration membranes at various pH values are discussed.
THEORETICAL BACKGROUND The surface of substances becomes charged if the substances are brought into contact with a polar (i.e., aqueous) medium. Two of the possible charging mechanisms are the adsorption of ions or polyelectrolytes of charged macromolecular species and the dissociation of surface groups [6]. In addition to the mixing tendency of thermal motion, these surface electric charges influence ions in the polar medium nearby in such a way that an electric double layer is formed [13]. This electric double layer consists of the charged surface and a diffuse region with an excess of counter-ions over coions (processing unlike and like charges to the surface charges). Due to the electroneutrality of the system, these surface charges are compensated by the charges of ions (counter-ions) in the diffuse part [13]. During a streaming current measurement on a microfiltration membrane, an electrolyte is forced by a pressure difference to flow through the pores of the membrane. In this case the electric double layer in the boundary phase of the surface of the membrane pores and the solution will be sheared. The net charge flow (streaming current I,) of ions contained in the electric
273
double layer can be recorded by two electrodes connected to an amperemeter and can be converted into the electric potential (zeta-potential C) at the surface of shear which has a distinct distance to the charged membrane surface. For the case of a laminar flow through a single circular capillary with diameter d and length 1, the equation which describes the relationship between the streaming current I, and the zeta-potential C can be written in the form [13]: (1) where E,, is the permitivity of the vacuum, e, the relative dielectric constant of the solution, Ap the applied pressure difference between the ends of the circular capillary and 11the viscosity of medium. As the volumetric flow rate 3 with the average velocity urn through the capillary obeys the Poiseuille’s equation [ 131, Eqn. (1) can be rewritten as follows [ 141: I, = -8xe,e,Cu,
32e,e,C
.
=-TV
(2)
Eqn. (2) can also be applied to a microfiltration membrane if the membrane is regarded as a bundle of circular capillaries with an average pore size that is larger than the thickness of the electric double layer appearing in the membrane pores.
EXPERIMENTAL
Principle of the measurement Since no electrode is completely reversible to any electrolyte, the charge flow through the microfiltration membrane, compared to the charge flow purely generated by the pressure difference, is more or less reduced by an electrode polarization occurring at the two electrodes employed in a normal streaming current measurement. For this reason the results of the normal streaming current measurements are in most cases falsified. However, utilizing the electrode polarization, we can obtain the exact streaming current by means of a four-electrode technique [ 111.The arrangement of the four electrodes employed in the improved streaming current measurement is shown schematically in Fig. 1.
274
El
Fig. 1. The electrode arrangement in the improved streaming current measurement: El, El’, the electrodes for potential measurement; E2, E2’, the electrodes for current measurement; M, the microfiltration membrane; C, the measuring cell; 0, O-ring seal.
Just like in the normal streaming current measurements, the two inner electrodes (E2, E2’) neighboring the membrane are used for the current measurement. The electrode polarization occurring at the two inner electrodes is observed with the two external electrodes (El, El’) by means of potential measurement. The charges of the ion current generated by the pressure difference are either digested by the electrodes (E2, E2’) or conductively transported backwards by the electric potential at the electrodes (E2,E2’) through the membrane. In the case of an application of an abrupt and constant pressure difference Apl to the systems, the temporal development of the measured quantities I and U is demonstrated in Fig. 2. The electric current recorded by the inner electrodes (E2, E2’) is reduced gradually from the streaming current while the electric potential measured by the external electrodes is increased continuously. Concerning the measured quantities I and U, the differences between the ideal case (streaming current measurement without electrode polarization) and the real case (streaming current measurement with electrode polarization) will meet finally to the constant values 1,-I.. and U&J_. The relationship between the time-dependent values I and U can be expressed by the equation of thermodynamics of irreversible processes [ 111:
275 I = LIAp-LzU
(3)
where L, and L, are the phenomenological
0 tl
coefficients.
t
Fig. 2. The temporal development of the measured quantities in an improved streaming current measurement: IO, I and I, - electric current flow through the electrodes (E2, E2’) at time 0, t and a~; U,, U and U, - electric potential at the electrodes (E2, E2’) at time 0, t and -; Zs - streaming current, if Z,-,is defined as the “zero” current. I ls2\
uo
U
Fig. 3. Linear relationship between timedependent Z and U under two pressure differences Apl and Apz. I0 and U, - electric current and potential at the time of 0; Is1 and Zs2 - streaming currents under pressure Ap1 and Ap2.
276
If the measured values of the current I are plotted down continuously vs. the measured values of the potential Uduring a measurement, a straight line will be obtained (Fig. 3). The extrapolation of one of the straight lines to the “zero” potential (U,) yields the streaming current I, with the classical definition [ 111.A change of the values of the applied pressure difference from Apl to Ap2 causes only a parallel line against the straight line obtained under the pressure difference APl* Materialsand chemicals AU microfiltration membranes used in this study had a nominal pore size of 0.2 pm. They were Millipore GVWP (polyvinylidenfluoride), Brunswick BTSD (polysulfone), Sartorius SMllO (cellulose acetate), Sartorius SM200 (nylon 66), Pall Ultipor NR (nylon 66) and Pall Posidyne NAZ (modified nylon 66). In order to obtain the same thickness of the electric double layer in the boundary phase of a membrane surface and the electrolytes, we kept the cation and anion concentration of the electrolytes used at lOA mol/l in the scope of pH 4 to pH 10. We prepared the electrolytes by use of distilled water adjusted with HCl, NaOH and HCl. Apparatus The apparatus for the improved streaming current measurement is shown in Fig. 4. A microfiltration membrane (M) was placed between two platinized platinum mesh electrodes in a plexiglass measuring cell (C) which was connected to two electrolyte reservoirs (R). In the electrolyte reservoirs there were two platinized plate electrodes. The surface of the mesh electrodes should be treated in such a way that in the electrolytes used an electrode polarizations at these electrodes occurred at a certain time during streaming current measurement. If a pressure difference was applied to the electrolyte reservoirs, the electrolyte flowed through the membrane. The values of the current and the potential measured by the four electrodes were amplified and recorded by an X-Y plotter (PL). The electrolyte flux through the membrane due to the pressure difference is determined by a balance (W) after each streaming current measurement. Before a streaming current measurement was started, the membrane was first degassed and then swelled in the electrolyte outside the cell for 24 h.
277
After that the membrane was washed for 20 min under a pressure difference of 0.1 bar in the cell by filtering the electrolyte through it.
CA
Fig. 4. Streaming current apparatus: A, amplifiers; PL, X-Y plotter; V, voltmeter; M, membrane; CA, compressed air; P, pressure sensor; E, electrodes; B, buffer; C, measuring cell; W, balance; RC, recorder.
All streaming current measurements were accomplished by pressure differences under 0.12 bar. During a measurement, the linear proportionality of the streaming current I, and the volumetric flow rate V of the electrolyte to the applied pressure difference Ap was checked for the membrane used.
RESULTS AND DISCUSSION
As the microfiltration membranes used had different structures with regard to the membrane porosities, the shapes of the membrane pores and the structures across a membrane thickness, it is very difficult to find the correct pore sizes of the membranes to evaluate the membrane surface potential (zeta-potential C) according to Eqn. (2). The surface charge properties of the microfiltration membranes were, therefore, characterized simply by the quotient of the measured streaming current I, and the volumetric flow rate 3 of the electrolytes.
278
Fig. 5 shows the determined quotients I,lvplotted vs. the pH values of the electrolytes for the cellulose acetate membrane (Sartorius SMllO) and the two native nylon 66 membranes (Pall Ultipor NR and Sartorius SM200).
4
5
6
7
8
9
10
PH
Fig. 5. pH influences on the surface charge of three membranes. a - Sartorius SMllO; b - Pall Ultipor NR; c - Sartorius SM200; Electrolyte concentration ccatioda~on = 10s4 mol/l.
The pH impact on the surface charge of these three membranes can be explained by the reactions of the membrane materials with the electrolytes if the ion adsorption on the membrane surfaces is negligible. On the surface of the cellulose acetate membrane (Sartorius SMllO), there were probably some groups which were able to dissociate as a weak acidic group. The number of these groups might be influenced by the hydrolysis of cellulose acetate. Thus the surface charge of the cellulose acetate membrane increased with the rise of the pH value of the electrolytes. At the high pH all weak acidic groups of the cellulose acetate membrane were dissociated, and, therefore, its surface potential remained constant. This effect has also been discussed theoretically by Demisch and Pusch [ 151.Their calculations of the surface charge of cellulose acetate membranes in the range of pH 4-10 delivered results that showed the same development as our experimentally determined results. The surface charge property of two native nylon 66 membranes (Pall Ultipor NR and Sartorius SM200) was much more influenced by the pH value of the electrolytes because many amino groups and carboxyl groups exist on the membrane surfaces. At a high pH value the surface charge of the membranes was mainly caused by the dissociation of the carboxyl groups which acted in the solutions as a weak acidic group. If the pH value of the electrolytes were decreased, the dissociation of the carboxyl groups dimin-
279
ished and the protonation of the amino groups increased. Therefore, with the decrease of the pH value the surface potential of the two native nylon 66 membranes became more positively charged that the enhanced negative values of the quotients IJvwere recorded. The similar pH influence on the surface potential of protein was reported by Shaw [13]. The difference of the surface charge properties between the Sartorius SM200 and Pall Ultipor might be dependent on the different ratios of the ammo groups to carboxyl groups existing on the surfaces of the two microfiltration membranes. Fig. 6 shows the results obtained by the streaming current measurements on the membranes which were modified. The modifications were made either on the membrane material (Pall Posidyne NAZ) or on the membrane surface (Millipore GV,WP and Brunswick BTSD). 750 600 GO
-600
I I L
5
6
7
6
9
l0
PH
Fig. 6. pH influences on the surface charge of three membranes: d - Millipore GVWP; e Brunswick BTSD; f - Pall Posidyne NAZ; Electrolyte concentration Ccation/anion= lob4 mol/l.
The typical amphoteric property of nylon 66 membrane could be quite well observed at the Pall Posidyne NAZ membranes whose surface potential kept positively charged in the whole investigated pH range by building of some chemical end groups in this membrane. These chemical end groups could be protonated to the quaternary ammonium end groups and enhanced the number of positively charged end groups on the nylon 66 membrane surface. The surfaces of the membranes made of hydrophobic materials, such as the Millipore GVWP and Brunswick BTSD membrane were always modified to improve the wettability of the membranes. As the hydrophobic materials do not have any ionic end groups which may cause surface charge of these membranes, the reason for the measured surface charge properties can only
be explained as a result of the ion adsorption on the membrane surface [16] and/or the reaction of the wetting agents with the electrolytes. The latter case is more convincing since positively and negatively charged polyvinylidenfluoride membranes have been offered by a membrane manufacturer at the same time. The pH dependence of the surface charge of the polysulfone membranes (Brunswick BTSD) is similar to the results reported for other polysulfone membranes [6,8]. Unfortunately, in one case [6] different chemicals besides HCl and NaOH were used to adjust different pH values, whereas in another case [B] the ion concentration of the electrolytes used was changed in several orders. For this reason a comparison of our results with those measured values is impossible. Although it was assumed by both authors that the ion adsorption, for example the adsorption of hydroxyl ions, is the main cause for the surface potential of polysulfone, we do not exclude other possibilities to impact the surface potential of the polysulfone membranes. One such possibility is the reaction of the surface modification agents (the wetting agents) with the electrolytes used. This might be the main reason for the negative surface charge of the polyvinylidenfluoride membrane (!&llipore GVWP). The existence of such reactions on the surface of this membrane in the electrolytes with a certain pH value was not denied by the membrane manufacturer. Nevertheless, we believe that the behavior of the surface charge of these two membranes in the electrolytes with various pH values should be an outcome of the combination of the ion adsorption and the reaction of the wetting agents on the membrane surface with the electrolytes. A general description of the surface charge properties of membranes made of such materials could hardly be made; because of the use of different wetting agents, their surface charges can be changed in different ways.
CONCLUSIONS
As we used the improved streaming current method, the surface charge properties of the synthetic microfiltration membranes were described by the measured quantities. Compared to the results of the normal streaming current measurements, the results of these improved streaming current measurements were not falsified by the electrode polarization. The shown pH dependency of the surface charge properties of the cellulose acetate membrane and the nylon 66 membranes could be explained by the pH influence on the dissociation and protonation of the surface end groups of the membrane materials. On the contrary, the surface reaction of the surface
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modification agents,i.e., the wetting agents, with the electrolytes and the ion adsorption on the membrane surface were probably the main causes of the surface charge of the nonionic polymer membranes in the electrolytes with various pH values.
ACKNOWLEDGEMENT
The authors wish to record their gratitude to the BMWi of the Federal Republic of Germany for financial support. This project is a part of the industrial joint research work of BMWi via DECHEMA.
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